Cryogenic air separation process and apparatus

ABSTRACT

An energy efficient process and apparatus for the cryogenic separation of air by rectification to produce at least one vapor fraction, at least one liquid fraction, and at least one nitrogen product stream wherein cooled and pressurized feed air in vapor form is condensed by indirect heat exchange contact with at least one liquid fraction to vaporize the liquid fraction and condense the feed air stream, then vaporizing the condensed feed air stream by indirect heat exchange contact with at least one vapor fraction thereby condensing the vapor fraction, and then using the vaporized feed air stream as feed air for cryogenic separation by rectification.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the field of air separation processes andparticularly to a process and apparatus for the production of nitrogen,oxygen and/or argon from air wherein liquefied air is used as the heatexchange medium for the high pressure column condenser to provide anenergy efficient process.

BACKGROUND ART

Standard cryogenic air separation processes involve filtering of feedair to remove particulate matter followed by compression of the air tosupply energy for separation. Generally the feed air stream is thencooled and passed through absorbents to remove contaminants such ascarbon dioxide and water vapor. The resulting stream is subjected tocryogenic distillation.

Cryogenic distillation or air separation includes feeding the highpressure air into one or more separation columns which are operated atcryogenic temperatures whereby the air components including oxygen,nitrogen, argon, and the rare gases can be separated by distillation.

Cryogenic separation processes involving vapor and liquid contact dependon the differences in vapor pressure for the respective components. Thecomponent having the higher vapor pressure, meaning that it is morevolatile or lower boiling, has a tendency to concentrate in the vaporphase. The component having the lower vapor pressure meaning that it isless volatile or higher boiling tends to concentrate in the liquidphase.

The separation process in which there is heating of a liquid mixture toconcentrate the volatile components in the vapor phase and the lessvolatile components in the liquid phase defines distillation. Partialcondensation is a separation process in which a vapor mixture is cooledto concentrate the volatile component or components in the vapor phaseand at the same time concentrate the less volatile component orcomponents in the liquid phase.

A process which combines successive partial vaporizations andcondensations involving countercurrent treatment of the vapor in liquidphases is called rectification or sometimes called continuousdistillation. The countercurrent contacting of the vapor and liquidphases is adiabatic and can include integral or differential contactbetween the phases.

Apparatus used to achieve separation processes utilizing the principlesof rectification to separate mixtures are often called rectificationcolumns, distillation columns, or fractionation columns.

When used herein and in the claims, the term "column" designates adistillation or fractionation column or zone. It can also be describedas a contacting column or zone wherein liquid or vapor phases arecountercurrently contacted for purposes of separating a fluid mixture.By way of example this would include contacting of the vapor and liquidphases on a series of vertically spaced trays or plates which are oftenperforated and corrugated and which extend crosswise of the column,perpendicular to the central axis. In place of the trays or plates therecan be used packing elements to fill the column.

"Double column" as used herein refers to a higher pressure column havingits upper end in heat exchange relation with the lower end of a lowerpressure column.

The term "a standard air separation process or apparatus" as used hereinis meant to describe that process and apparatus as above described aswell as other air separation processes well known to those skilled inthe art.

As used herein and in the appended claims, the term "indirect heatexchange" means the bringing of two fluid streams into heat exchangerelation without any physical contact or intermixing of the fluids witheach other.

Historically, nitrogen, oxygen and/or argon have been produced by one oftwo basic process schemes including the single column process and thedouble column process.

With respect to nitrogen, the single column process produces goodquality gaseous and liquid nitrogen at pressures of approximately 6-10bar. The recovery of nitrogen is limited by the equilibrium at thebottom of the column. Typically, the process can produce nitrogen at arate of approximately 50-60% of the nitrogen in the initial air feed.

With the double column process, nitrogen is produced at pressures ofabout 1-4 bar. It is more efficient than the single column process, andapproximately 90% or more of nitrogen can be recovered from the nitrogenpresent in the initial air feed. Typically the columns are stacked witha condenser-reboiler separating the two columns. Since the processproduces nitrogen at relatively low pressures, further compression ofnitrogen is frequently needed adding to the cost of production and use.

In the prior art double column process, air is separated by cryogenicdistillation or rectification to produce a nitrogen-rich stream orfraction at the top of the high pressure column and oxygen-rich streamor fraction at the bottom. The nitrogen-rich stream is sent to the topof the low pressure column to provide the reflux for this column. Thebottom oxygen-rich stream is fed to the low pressure column for furtherseparation.

In the low pressure column the feed stream is further separated bycryogenic distillation into an oxygen-rich stream or fraction at thebottom and a nitrogen-rich stream or fraction at the top. The top streamcan then be recovered as nitrogen product. In the double columnarrangement, the high pressure column and the low pressure column arethermally linked through the condenser-reboiler arrangement. Thus, inthe prior art double column process the nitrogen-rich fraction of thehigh pressure column is condensed against the vaporizing oxygen-richfraction of the low pressure column.

For a given pressure in the low pressure column, the pressure of the airfeed to the high pressure column is dictated by the composition of thevaporizing oxygen-enriched stream, the temperature difference of thehigh pressure column condenser and the low pressure column reboiler, andto some extent the composition of the condensing nitrogen-enrichedstream which is relatively pure in nitrogen.

Other prior art process schemes are variations of the above describedsingle or double column process with additional features such as anadditional overhead condenser or bottom reboiler.

SUMMARY OF THE INVENTION

The process of the invention can be utilized for the energy efficientproduction of nitrogen, oxygen and argon.

Essentially, the invention lies in using vaporized and liquefied air asthe heating and cooling medium between the high pressure and the lowpressure columns. Formerly nitrogen has been used.

The invention will be explained in particular detail with respect tonitrogen but it should be understood that the invention is equallyapplicable to the production of oxygen and argon. It will be obvious tothose skilled in the art how to optimize temperature, pressure and otheroperating conditions to optimize output of oxygen and/or argon asprimary product.

The particular advantage in the use of air for the heating and coolingmedium is that less energy is required to condense the air than tocondense a nitrogen rich stream. Since the main energy cost involvescompression of the gases, the lower pressure which is required tocondense air at a given temperature is less costly than to condensenitrogen.

For example, nitrogen condenses at 7 bar pressure at -180° C. Bycontrast, only 6 bar pressure at -178° C. is required to condense air.Thus the 2° C. difference in temperature and the 1 bar pressure providesthe reduced energy expenditure in the invention process.

In prior art processes wherein nitrogen is used for the heating andcooling medium between the high pressure and low pressure columns, it isnecessary to compress the feed air to a higher feed air pressure asrequired by the nitrogen. Thus, the primary energy savings come from thereduced requirement for compression of the feed air.

The process of the invention makes possible the production of highpurity nitrogen to the extent of more than 90% of the nitrogen containedin the initial feed air. It can be produced at a pressure range withinabout 3 bar to about 15 bar. Both high pressure and low pressurenitrogen can be produced. This can be done separately or together.Moreover, the process is energy efficient compared with prior artprocesses.

According to the invention process, feed air, which has been treated toremove moisture and impurities such as CO₂ and methane by passagethrough molecular sieves, alumina, silica gel and the like is compressedand fed to a heat exchanger to exchange heat with outgoing products.

According to one embodiment, the feed air is split into two fractions,one fraction being fed to the bottom of a high pressure column and theother fraction being fed to a condenser/reboiler located in the base ofa low pressure column. Good results have been obtained by using equalfractions of feed air although other ratios can be used.

According to another embodiment, the feed air is split into threefractions. Two of the feed air fractions are fed to the high pressurecolumn and the condenser/reboiler at the base of the low pressure columnas above described. The third air fraction is expanded to provide plantcooling and then introduced into the low pressure column for cryogenicseparation.

The first feed air fraction is separated by cryogenic distillationwithin the high pressure column into a first nitrogen-rich vaporfraction and a first oxygen-rich liquid fraction. The oxygen enrichedliquid fraction is withdrawn from the base of the high pressure columnand sent to the low pressure column. The second feed air fraction whichis sent to the condenser/reboiler in the base of the low pressure columnis condensed by heat exchange with the oxygen-rich liquid at the bottomof the low pressure column which is thereby vaporized. The condensedliquefied air thus produced in the condenser/reboiler is then fed to thetop condenser of the high pressure column where it is vaporized byindirect heat exchange with the first nitrogen-rich vapor fractionproduced in the high pressure column. This causes the nitrogen tocondense.

According to one embodiment, part of the condensed nitrogen-richfraction in the high pressure column is separated and fed to the lowpressure column to provide extra reflux. At the same time the secondfeed air fraction which has been vaporized by indirect heat exchangecontact with nitrogen in the top condenser of the high pressure columnis then introduced into the low pressure column for cryogenicseparation.

Within the low pressure column, the second feed air fraction along witha portion of the first oxygen-rich fraction from the high pressurecolumn are then separated into a second nitrogen-rich stream and asecond oxygen-rich stream.

According to another embodiment, a portion of the second nitrogen-richstream can be removed as high pressure nitrogen product while theremaining portion is used to provide reflux for the low pressure column.

According to another embodiment, a portion of the high pressure nitrogenproduct can be expanded to provide plant cooling and added to the lowpressure nitrogen product stream.

The second oxygen-rich stream which falls to the bottom of the lowpressure column is vaporized by indirect heat exchange contact with theincoming second feed air fraction which is thereby condensed. By anotherembodiment, the second oxygen-rich fraction can also include a thirdfeed air fraction which has been expanded prior to being introduced intothe low pressure column.

A portion of the second oxygen-rich stream is fed to the overheadcondenser of the low pressure column where it is vaporized by heatexchange contact with rising nitrogen which is thereby condensed. Thethus vaporized second oxygen-rich stream can be removed from theoverhead condenser as waste and warmed in subcoolers and in the heatexchanger by indirect heat exchange with process streams and feed air.

If desired the waste oxygen can be expanded to provide plant cooling.Alternately, the waste oxygen which has about 70% purity can be utilizedas product in applications where high purity oxygen is not required.

Apparatus for the above described process are also provided. Theapparatus include, in combination, air compression means for compressingair from an outside source, purification means for removing carbondioxide and water vapor from the air compressed by the air compressionmeans, and heat exchange means for cooling the compressed air from thepurification means to a cryogenic temperature. A first distillationcolumn equipped with a top column or overhead evaporator/condenser isincluded for cryogenic separation of a portion of the feed air from theheat exchanger.

A second distillation column equipped with a top column condenser and abottom column reboiler is provided for separation by fractionation of atleast a portion of the cooled compressed feed air after circulationthrough the bottom column reboiler of the second distillation column andthe top column condenser of the first distillation column together withat least a portion of the oxygen-rich liquid obtained from the firstdistillation column into a second oxygen-rich fraction and a secondnitrogen-rich fraction.

Means are provided for withdrawal of oxygen liquid at the base of thesecond distillation column for introduction into the overhead condenserof the second distillation column to provide indirect heat exchange withvapors rising within the second distillation column.

Expansion means are provided for expansion of compressed air prior tointroduction in the second distillation column, of oxygen withdrawn fromthe overhead condenser of the second distillation column, and/or forexpansion of nitrogen product to provide cooling.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic flow diagram of the process and apparatus ofthe invention in which low pressure nitrogen is produced;

FIG. 2 shows a schematic flow diagram of the process and apparatus ofthe invention similar to FIG. 1 except that air expansion is provided inplace of waste expansion;

FIG. 3 shows a schematic flow diagram of the process and apparatus ofthe invention wherein high pressure and low pressure nitrogen areproduced; and,

FIG. 4 shows a schematic flow diagram of the process and apparatus ofthe invention similar to FIG. 3 wherein part of the high pressurenitrogen is expanded to low pressure nitrogen.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the flow diagram of FIG. 1, compressed feed air free ofimpurities is introduced by means of conduit 20 into a heat exchanger30. The air is preferably introduced into the heat exchanger 30 at apressure in the range of about 5 bar to about 20 bar where thetemperature of the air is cooled to cryogenic temperature by indirectheat exchange with outgoing waste and product streams.

Next the feed air is split into two fractions. Good results have beenobtained with equal fractions or streams of feed air but other ratioscan be used. The first fraction of the feed air is sent to the highpressure column 32 through lines 22 and 62 and the remaining secondfraction of feed air is sent to the reboiler 58 of the low pressurecolumn 34 through lines 22 and 60.

At the high pressure column 32 the pressure is preferably in the rangeof about 5 bar to 20 bar.

The first feed air fraction is introduced into the lower part of column32 below the bottom distillation tray as indicated at 36. Here, thefirst feed air fraction is separated into a first nitrogen-rich vaporfraction which rises to the top of the column 32 and a first oxygen-richliquid fraction which falls to the bottom of the column 32.

At least a portion of the first oxygen-rich liquid is withdrawn from thebottom of the high pressure column at 38. It is comprised of about 35%to about 40% oxygen which is about the same proportion as for the priorart processes.

The first oxygen-rich liquid which is removed from the bottom of thehigh pressure column 32 through line 54 is passed through subcooler 46where the temperature is further reduced by indirect heat exchange withproduct nitrogen which exits from the upper part of the low pressurecolumn 34 through line 48 and with waste which exits through line 52from the overhead condenser/evaporator 70 of the low pressure column 34.

The cooled first oxygen rich liquid from the subcooler 46 is thenintroduced into the low pressure column 34 above the bottom tray afterexpansion through valve 76.

The second feed air fraction which enters the condenser/reboiler 58 inthe base of the low pressure column 34 is condensed by indirect heatexchange with oxygen-rich liquid at the bottom of the low pressurecolumn 34. This causes the second feed air fraction to be condensed andthe oxygen-rich liquid to be vaporized.

The condensed second feed air fraction leaves the condenser/reboiler 58of the low pressure column 34 via line 82 where it enters subcooler 46.The liquefied air exits subcooler 46 via line 84 and expands throughvalve 44 into the condenser/reboiler 40 of the high pressure column 32.If needed, a portion of the condensed second feed air fraction can beintroduced into the low pressure column 34 via line 90 after expansionthrough valve 92 to control the balance of air between the high pressureand low pressure columns.

The first nitrogen-rich vapor fraction rises to the top of the highpressure column 32 where it enters the condenser/reboiler 40. Here thenitrogen vapor is brought into indirect heat exchange contact with thecondensed second feed air fraction which enters through valve 44 fromthe condenser/reboiler 58 of the low pressure column 34. This causes theliquefied air to vaporize and the nitrogen vapor to be condensed. Asshown in FIGS. 3 and 4, part or all of the condensed nitrogen portion isreturned to the high pressure column 32 to provide reflux as required.

Any nitrogen vapor which is not condensed by indirect heat exchange withthe condensed second feed air fraction can be recovered as high pressurenitrogen by removal from the upper part of the high pressure column 32for example, through line 67 as shown in FIG. 3.

Part of the condensed nitrogen can be sent to the low pressure column 34for extra reflux if the high pressure nitrogen flow is small or notneeded. This part of the condensed nitrogen is removed from the upperpart of the high pressure column 32 through line 68 as shown in FIGS. 1and 3. The condensed nitrogen is then passed through subcooler 66 whereit is brought into indirect heat exchange contact with outgoing nitrogenproduct and waste. From the subcooler 66, the condensed nitrogen passesthrough a continuation of line 68 and is introduced into the lowpressure column 34 after expansion through valve 78.

At the same time, the vaporized air exiting via line 56 from thecondenser/reboiler 40 at the top of the high pressure column 32 isseparated by introduction into the low pressure column 34 through line64 at about the same level as for the introduction of the firstoxygen-rich liquid which enters through line 54.

The first oxygen-rich liquid withdrawn from the base of column 32 andthe vaporized air withdrawn from the condenser/reboiler 40 at the top ofthe high pressure column 32 through line 56 are further separated withincolumn 34 into a second nitrogen-rich vapor fraction and a secondoxygen-rich fraction.

The second nitrogen-rich vapor fraction rises to the top of the lowpressure column 34 while the second oxygen-rich fraction falls to thebottom of the low pressure column 34.

A portion of the second oxygen-enriched liquid fraction at the bottom ofthe low pressure column 34 is withdrawn through line 74 and passedthrough a first subcooler 46. Here the second oxygen-enriched liquid isfurther cooled by indirect heat exchange with nitrogen gas removed fromthe upper part of the low pressure column 34 through line 48 and withthe waste stream exiting through line 52 from the overhead condenser 70of the low pressure column 34.

The second oxygen-enriched liquid is passed by means of a continuationof line 74 to a second subcooler 66 for further cooling by indirect heatexchange with nitrogen gas removed from the top of the high pressurecolumn 32 through line 68 and with the waste oxygen stream which exitsfrom the overhead condenser 70 through line 52.

The resulting cooled second oxygen-rich liquid is passed through anextension of line 74 where the liquid is introduced into the overheadcondenser 70 in the top of the low pressure column 34 after expansionthrough a valve 72 to further cool the second oxygen enriched stream.

A major part of the second nitrogen-rich stream is recovered as nitrogenproduct from the upper part of the low pressure column 34 through line48. The gaseous nitrogen stream is warmed by passage through subcoolers66 and 46 and heat exchanger 30 before exiting the system.

The remaining portion of the second nitrogen-rich stream within the lowpressure column 34 is condensed by heat exchange with the secondoxygen-enriched liquid in the overhead evaporator/condenser 70 of thelow pressure column 34 which causes the second oxygen-enriched liquid tobe vaporized. The condensation of the nitrogen provides reflux for thelow pressure column 34. The vaporizing oxygen-enriched liquid exitsoverhead evaporator/condenser 70 via line 52 and is subsequently warmedby passage through subcoolers 66 and 46 and heat exchanger 30.

After warming in the heat exchanger 30, the waste oxygen stream ispassed through a turbo expander 78 where the stream can be expanded toprovide plant cooling.

It can seen that the above described process utilizes air as a heatingand cooling medium between the high pressure and low pressure columns.Conventionally in prior art processes, the nitrogen-rich stream has beenused to transfer heat to the bottom of the low pressure column. Keepingin mind that for a given nitrogen recovery, that is, having the samecomposition of oxygen-rich stream, more energy is required to condensethe nitrogen-rich stream than to condense air. What this means is thatfor a given nitrogen recovery, using air as the heat transfer medium,the high pressure column can function at a lower pressure than forconventional prior art processes. Also, for the same pressure in thehigh pressure column, according to the invention process, the lowpressure column can function at a higher pressure.

Table 1 below shows the expected performance of the invention processshown in FIG. 1 and above described for the products of nitrogen asproduct.

                  TABLE 1                                                         ______________________________________                                        Total Feed Air Flow Line 20  15462 Nm.sup.3 /h                                Feed Air Pressure   Line 20  10.2 bar abs.                                    Nitrogen Product Flow                                                                             Line 48  10514 Nm.sup.3 /h                                Nitrogen Pressure   Line 18  5.5 bar abs.                                     Nitrogen Purity              18 vpm 02                                        Waste (Oxygen-Rich) Flow                                                                          Line 52  4948 Nm.sup.3 /h                                 Waste Pressure      Line 16  1.3 bar abs.                                     Compressed Air      Line 22  -160° C.                                  Column 32                    10.2 bar abs.                                    Column 32           Top      -170° C.                                  Column 32           Bottom   -160° C.                                  Oxygen-Rich Liquid  Line 38  -165.6° C.                                Condensed Second Feed Air                                                                         Line 82  -167.5° C.                                Fraction                                                                      Condensed Second Feed Air                                                                         Line 82  -167.5° C.                                Fraction                                                                      Condensed Second Feed Air                                                                         Line 84  -171° C.                                  Fraction                                                                      Vaporized Second Feed Air                                                                         Line 56  -172.6° C.                                Fraction from Condenser/                                                      Reboiler 40                                                                   Nitrogen Exiting Column 32                                                                        Line 68  -170.6° C.                                Condensed Nitrogen Exiting                                                                        Line 68  -174.4° C.                                Subcooler 66                                                                  Column 34                    5.5. bar abs.                                    Oxygen-Rich Liquid from Column                                                                    Line 74  -168.8° C.                                34                                                                            Oxygen-Rich Liquid Exiting from                                                                   Line 74  -174.4° C.                                Cooler 66                                                                     Oxygen-Rich Liquid after                                                                          Valve 72 -179° C.                                  Expansion                                                                     Nitrogen Product Exiting Column                                                                   Line 48  -177.6° C.                                34                                                                            Nitrogen Product Exiting Column                                                                   Line 48  5 bar abs.                                       34                                                                            Oxygen Waste Stream from                                                                          Line 52  -178.5° C.                                Condenser 70                                                                  ______________________________________                                    

When the embodiment shown in FIG. 3 or FIG. 4 is followed, a feed airpressure of 21 bar abs. would produce a pressure of about 20 bar abs.within the high pressure column 32 and a pressure of about 14 bar abs.within the low pressure column 34.

Various modifications of the invention process and apparatus as abovedescribed will be apparent to those skilled in the art and can beresorted to without departing from the spirit and scope of the inventionas defined by the following appended claims.

I claim:
 1. In a process for the cryogenic separation of air byrectification in at least one distillation column to produce at leastone vapor fraction, at least one liquid fraction, and at least oneproduct nitrogen stream the improvement comprising:providing a cooledand pressurized feed air stream in vapor form; condensing at least aportion of said feed air vapor stream by indirect heat exchange contactwith at least one of said liquid fractions to vaporize said liquidfraction and condense said feed air stream; and vaporizing a portion ofsaid condensed feed air stream by indirect heat exchange contact withone of said vapor fractions thereby condensing said vapor fraction. 2.The process of claim 1 wherein said vaporized feed air stream is furtherintroduced into one of said distillation columns as feed air forcryogenic separation by rectification.
 3. In a process for producingnitrogen by cryogenic distillation of air in a high pressuredistillation column to produce a first oxygen-rich fraction and a firstnitrogen-rich liquid fraction; and introducing at least a portion ofsaid first oxygen-rich fraction into a low pressure distillation columnto produce a second oxygen-rich fraction and a second nitrogen-richfraction by cryogenic distillation, the improvement whichcomprises:bringing cooled feed air into indirect heat exchange contactwith said second oxygen-rich fraction to vaporize at least a portion ofsaid second oxygen-rich fraction and condense at least a portion of saidair; bringing said condensed portion of said feed air into indirect heatexchange contact with said first nitrogen-rich fraction to condense saidfirst nitrogen-rich fraction and to vaporize said condensed portion ofsaid feed air; and, introducing said vaporized portion of said feed airinto said low pressure distillation column for cryogenic separation. 4.A process as claimed in claim 3 further comprising:separating feed airinto a first feed air fraction which is introduced into said highpressure column for cryogenic separation and a second feed air fractionwhich is brought into indirect heat exchange relation with said secondoxygen-rich fraction to vaporize at least a portion of said secondoxygen-rich fraction and condense at least a portion of said second feedair fraction; passing said condensed second feed air fraction intoindirect heat exchange relation with said first nitrogen-rich fractionto condense said first nitrogen rich fraction and vaporize saidcondensed second feed air fraction; and, introducing said vaporizedsecond feed air fraction into said low pressure column for cryogenicseparation.
 5. A cryogenic process for producing nitrogen from aircomprising:A) Dividing cooled compressed feed air substantially free ofmoisture and impurities into a first feed air fraction and a second feedair fraction; B) Feeding said first feed air fraction into a highpressure column equipped with a top condenser; C) Separating said firstfeed air fraction within said high pressure column by cryogenicdistillation into a first nitrogen-rich fraction and a first oxygen-richfraction; D) Withdrawing at least a protion of said first oxygen-richfraction from said high pressure column; E) Introducing at least aportion of said first oxygen-rich fraction into a low pressure columnequipped with a bottom condenser/reboiler and an overheadevaporator/condenser for cryogenic separation into a secondnitrogen-rich fraction and a second oxygen-rich fraction; F)Introduccing said second feed air fraction into said consenser/reboilerin said low pressure column; G) Condensing said second feed air fractionby indirect heat exchange with said second oxygen-rich fraction in saidlow pressure column thereby vaporizing at least a portion of said secondoxygen-rich fraction; H) Introducing at least a portion of saidcondensed second feed air fraction into said top condenser of said highpressure column; I) Vaporizing at least a portion of said secondcondensed feed air fraction within said top condenser of said highpressure column by indirect heat exchange with at least a portion ofsaid first nitrogen-rich fraction in said high pressure column tocondense at least a portion of said first nitrogen-rich fraction; J)Introducing into said low pressure column at least a portion of saidsecond feed air fraction vaporized by indirect heat exchange contactwith said first nitrogen-rich fraction in said top condenser of saidhigh pressure column for cryogenic separation together with at least aportion of said first oxygen-rich fraction into a second nitrogen-richfraction and a second oxygen-rich fraction; K) Removing at least aportion of said second nitrogen-rich fraction as product from said lowpressure column; L) Withdrawing at least a portion of said condensedsecond oxygen-rich fraction from said low pressure column; M)Introducing at least a portion of said withdrawn oxygen-rich fractioninto said overhead condenser of said low pressure column; N) Vaporizingat least a portion of said second oxygen-rich fraction in said overheadcondenser by indirect heat exchange with at least a portion of saidrising second nitrogen-rich fraction within said low pressure columnthereby causing said second nitrogen-rich fraction to be condensed andproviding reflux for said low pressure column; and, O) Withdrawing atleast a portion of said vaporized second oxygen-rich fraction from saidoverhead condenser as waste.
 6. A process as claimed in claim 5 furthercomprising:withdrawing at least a portion of said condensed firstnitrogen-rich fraction from said high pressure column as high pressurenitrogen product.
 7. A process as claimed in claim 6 furthercomprising:expanding at least a portion of said waste oxygen withdrawnfrom said overhead condenser to provide plant cooling.
 8. A process asclaimed in claim 6 further comprising:expanding at least a portion ofsaid high pressure nitrogen product prior to discharge with said lowpressure nitrogen product.
 9. A process as claimed in claim 5 furthercomprising:withdrawing at least a portion of said condensed firstnitrogen-rich fraction from said high pressure column; and, introducingat least a portion of said withdrawn condensed first nitrogen-richfraction into said low pressure column.
 10. A process as claimed inclaim 5 further comprising:further dividing said compressed feed airinto a third feed air fraction; expanding at least a portion of saidthird feed air fraction to provide cooling; and, introducing at least aportion of said expanded feed air fraction into said low pressurecolumn.
 11. A process as claimed in claim 5 further comprising:coolingsaid feed air by indirect heat exchange contact with waste and productstreams; and, compressing said feed air to provide a pressure in thehigh pressure column in the range of about 2 bar to about 20 bar.
 12. Aprocess as claimed in claim 5 wherein:said first feed air fraction instep B) is fed into the lower half of said high pressure column; and,said first oxygen-rich fraction in step D) is withdrawn from the base ofsaid high pressure column.
 13. A process as claimed in claim 5wherein:said first oxygen-rich fraction of step E) is introduced intothe lower half of said low pressure column; and, said second oxygen-richfraction of step N) is withdrawn from the base of said low pressurecolumn.
 14. A process as claimed in claim 5 further comprising:passingsaid waste oxygen obtained in step Q) through a turbo expander toprovide cooling; and warming said cooled waste oxygen from said turboexpander by indirect heat exchange contact with feed air which isthereby cooled.
 15. Apparatus for producing nitrogen from cooledcompressed air comprising:a first distillation column equipped with atop column condenser for cryogenic separation by fractionation of aportion of said cooled compressed feed air into a first nitrogen-richfraction and a first oxygen-rich fraction; a second distillation columnequipped with a top column condenser and a bottom column reboiler forseparation by fractionation of at least a portion of the cooledcompressed feed air after circulation through said bottom columnreboiler of said second distillation column and said top columncondenser of said first distillation column together with at least aportion of said first oxygen-rich obtained from said first distillationcolumn into a second oxygen-rich fraction and a second nitrogen-richfraction; conduit means within said first and said second distillationcolumns for the introduction and withdrawal of liquids and vapors;conduit means in communication between said first and said seconddistillation columns for introduction and withdrawal of liquids andvapors; conduit means in communication with said bottom column reboilerof said second distillation column for the introduction of cooledcompressed feed air; conduit means in communication with said bottomcolumn reboiler of said second distillation column and said top columncondenser of said first distillation column for transfer of condensedfeed air from said reboiler in said second distillation column to saidtop column condenser in said first distillation column; conduit means incommunication with said top column condenser of said first distillationcolumn and said second distillation column for withdrawal of vaporizedair from said top column condenser of said first distillation column andintroduction into said second distillation column for cryogenicseparation; and, conduit means in communication with said firstdistillation column and said second distillation column for withdrawalof at least a portion of said first oxygen-rich fraction from the bottomof said first distillation column and introduction into said seconddistillation column for cryogenic separation.
 16. Apparatus as claimedin claim 15 further comprising:conduit means in communication with saidsecond distillation column and said top column condenser of said seconddistillation column for withdrawal of said oxygen-rich fraction fromsaid second distillation column and introduction into said top columncondenser of said second distillation column to provide indirect heatexchange with vapors rising within said second distillation column;conduit means in communication with said top column condenser of saidsecond distillation column for withdrawal of said second oxygen-richfraction as waste; and, conduit means in communication with said firstdistillation column and said second distillation column for withdrawalof said first nitrogen-rich fraction from said first distillation columnand introduction into said second distillation column to provide refluxfor said second distillation column.
 17. Apparatus as claimed in claim16 further comprising:compression means for compressing air from anoutside source; purification means for removing carbon dioxide, watervapor and other impurities from air compressed by said air compressionmeans; heat exchange means for cooling the compressed air from saidpurification means to a cryogenic temperature; conduit means incommunication with said top column condenser of said second distillationcolumn for the introduction and withdrawal of liquids and vapors;conduit means in communication with said heat exchanger and said firstdistillation column and said second distillation column for theintroduction of cooled compressed feed air; and, valve means within atleast one of said conduit means for metering of vapors and liquids andfor expansion therethrough.
 18. Apparatus as claimed in claim 15 furthercomprising:conduit means in communication with said first distillationcolumn and said heat exchange means for withdrawal of nitrogen product.19. Apparatus as claimed in claim 18 further comprising:expansion meansin communication with said conduit means for expansion of at least aportion of nitrogen product to provide cooling.
 20. Apparatus as claimedin claim 15 further comprising:expansion means for expansion of oxygenwaste.
 21. An apparatus as claimed in claim 15 further comprisingexpansion means for expansion of cooled compressed air prior tointroduction into said second distillation column to provide cooling.